Philippine troops entered a besieged city on the southern island of Mindanao Thursday to begin clearing out Islamist militants who virtually took control earlier this week.

Two military helicopters flew above the city’s rooftops as armored tanks rolled through its neighborhoods amid sporadic gunfire and explosions. Many of Marawi’s 200,000 citizens are fleeing to safety.

The crisis began late Tuesday when Philippine security forces launched a mission to arrest militant leader Isnilon Hapilon, who was believed to be in Marawi recovering from wounds sustained in a previous clash. The raid collapsed after a wave of militants stormed the city and went on a rampage, burning houses, a university and Catholic churches and taking hostages, including a priest and more than a dozen others at a cathedral.

The military says at least six government troops and 13 militants have been killed in the fighting.

President Rodrigo Duterte has declared martial law in the southern Philippines, and has vowed to take harsh measures against the militants. He also warned he might expand martial law nationwide.

Duterte’s use of martial law has raised concerns among human rights groups, who have accused him of ordering security forces to kill thousands of people as part of his crackdown on illicit drugs.

The southern Philippines, particularly the resource-rich but poverty-wracked Mindanao region, has long been a hotbed of activity by the Abu Sayyaf and other fundamentalist groups.

His time as leader of the brutal Abu Sayyaf terror group, which has been blamed for numerous bombings, kidnappings for ransom and beheadings in the Philippines, includes the 2004 bombing of a ferry in Manila Bay that claimed more than 100 lives.

The United States has placed a $5 million bounty on his head for alleged terrorist acts against American citizens.